Method and apparatus for measuring lifetime of semiconductor material including waveguide tuning means

ABSTRACT

A method and apparatus measure the lifetime of semiconductor materials having resistivity values within a predetermined measurement range. Carriers are produced within the semiconductor material responsive to incident energy. Microwave energy is radiated from a waveguide onto the semiconductor material to obtain reflected microwave energy. The equivalent distribution circuit characteristics of the waveguide are varied such that a variation of a magnitude of the reflective microwave energy relative to a resistivity of the semiconductor material is substantially linear within the predetemined measurement range. Plural stub tuners are provided within the waveguide to obtain the desired linear characteristics. The lifetime measurement is obtained in accordance with an attenuation of the produced carriers.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional application of Ser. No. 07/475,768,filed Feb. 5, 1990, which issued as U.S. Pat. No. 5,081,414 on Jan. 14,1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for measuring the lifetime ofa semiconductor material and an apparatus therefor, and in particular,to a method and a device for determining the lifetime of thesemiconductor material in which pulse energy is injected into thesemiconductor material in order to generate a particular attenuation ofa carrier, and by measuring the attenuation characteristics of thecarrier, highly reliable and effective detection of a defectivesemiconductor material crystal and a very small heavy metal taint isenabled.

2. Description of the Prior Art

Semiconductor materials, such as Si, Ga and As, are processed accordingto several hundreds of processing steps, including raw materialpreparation steps through to semiconductor device manufacturing steps.These raw material preparation and the semiconductor devicemanufacturing steps include material washing, impurity diffusion,thermal treatment, patterning and etching, and each disadvantageouslyintroduces the possibility of the generation of defective crystals andheavy metal taints in the semiconductor material. Furthermore, since thenumber of such troublesome processing steps is very high, the control ormanagement of these steps, which necessitate many laborious and preciseoperation, is very difficult.

Nevertheless, it is inevitable that some semiconductor materials orchips having defective crystals and/or heavy metal taints or pollutantswill be produced at a certain rate. As a result, a number of devices foridentifying defective crystals and fine metal taints in semiconductormaterials or chips have been widely marketed. A general and conventionalprocess for measuring and analyzing defective crystals in thesemiconductor materials or chips has been used for determining therespective lifetimes of the chips.

FIG. 1 is a block diagram of the structure of a prior art apparatus formeasuring the lifetime of a semiconductor material.

As shown in FIG. 1, microwave energy generated by a microwave oscillator1 is guided by a magic tee 4 to a wave guide 8 via an impedance matchingdevice 2 and an E-H tuner 3, and is irradiated onto a semiconductormaterial 10 which is the object of the measurement. The microwave energya irradiated onto the semiconductor material 10 is reflected from thenear surface of the material 10, from within the material 10, and fromthe reverse surface of the material 10, respectively, and returns to themagic tee as microwave energy b which is guided by the magic tee 4 tothe E-H tuner 6 and detected by a detector 7.

The principle of the lifetime measurement will now be explainedreferring to FIG. 2, where the reference a denotes microwave energywhich is constantly applied to the semiconductor material 10. Carriersare operatively produced when external energy is fed to thesemiconductor material 10 in the form of pulses from a laser diode 9 ata time of the measurement. Since this area producing the carriers isequivalent to the semiconductor turning into a conductor, and since themicrowave energy is reflected from the produced carriers (microwaveenergy is reflected 100% on a metal), the reflection of the microwaveenergy is detected as a temporary increase in the region. Thechronological change of the increased microwave energy coincides withthe chronological attenuation waveform of the produced carriers.Therefore, the crystal in the semiconductor material 10 can be evaluatedby measuring the attenuation waveform (lifetime) of the producedcarriers.

The respective views FIGS. 3A to 3C show examples of microwaveirradiation set ups of a conventional apparatus for measuring thesemiconductor lifetime. In particular, FIG. 3A depicts a microwaveirradiation set up provided with a measurement table 21 made ofnon-metal material and a waveguide 8 for emitting and receiving themicrowave energy, and a semiconductor material 10 to be measured beingplaced on the measurement table 21. FIG. 3B shows another microwaveirradiation set up provided with the waveguide 8 described above and ameasurement holder 22 made of non-metal material on which asemiconductor material 10 to be measured is held, the measurement holder22 being used in place of the measurement table 21 shown in FIG. 3A.FIG. 3C shows still another example of a microwave irradiation set uphaving a metal plate 23 from which the microwave energy reflects, inaddition to the measurement table 21 having the semiconductor material10 placed thereon and the waveguide 8.

A contactless inspection method, such as a DLTS method (Deep LevelTransient Spectroscopy), has been widely employed to detect fine or verysmall metal taints in a semiconductor material. According to the DLTSmethod, a diode is formed on a substrate and a voltage is impressed onthe diode, generating a response to the impression after the voltage iscut off. The response as shown in FIG. 13D is measured or determined inthe form of a changed amount I in the electric signal between theinstants t₁ and t₂. The relation between the values of the changedamount I and temperature are plotted to determine the degree of metaltaints using particular thermal peak values of the changed amount Ibased on various metals.

Although the object materials of the lifetime measurement are usuallysemiconductor materials, such as a Si-wafer, the resistivity of thematerials ranges fairly extensively depending on the usage of devices.

The waveguide 8 which is used in the prior art lifetime measuringapparatus can be equivalently replaced with a distribution circuit asshown in FIG. 4A. If the distribution circuit is terminated with aterminal resistance Z₁, a reflected signal corresponding to the terminalresistance Z₁ is produced as shown in FIG. 4B. Accordingly, in the caseof the apparatus shown in FIG. 1 for example, the horizontal axis Z inFIG. 4B may be replaced with the resistivity ρ_(s) since the terminalresistance Z₁ is equivalent to the resistivity of the Si-wafer. Thesignal of the reflected microwave energy measured in the regions a, cand b corresponding to the resistivities ρ_(a), ρ_(o1) and ρ_(b) shownin FIG. 4B becomes as shown in FIGS. 5A, 5B and 5C, respectively. Theregion c is where the measurement is impossible. In other regions, suchas d, e and f, the reflected microwave signals to be measured aregreatly influenced by the non-linear characteristics of the abovementioned waveguide to thereby have significantly changed in signalintensity and deteriorated in data reproducibility. The prior art methodis problematic since it cannot measure some of the object materials witha high reliability.

From another standpoint the prior art is problematic in that accordingto the conventional process for analyzing a character of thesemiconductor material or for locating any defective crystals as shownin FIG. 3A, the effective amount of the reflected microwave signal issmall due to the effects of the microwave energy a' passingdisadvantageously through the semiconductor material 10 and themicrowave energy b' reflecting from the measurement table 21. Accordingto the conventional device as shown in FIG. 3B, the microwave energy bhas a small S/N ratio value because the microwave energy passes throughthe semiconductor material 10 and is reflected on the measurement holder22. In order to improve the S/N ratio of the signal by making themicrowave a' passing through the semiconductor material 10 reflected onthe metal plate 23 so as to increase the volume of the microwave breflected from the semiconductor material 10, the device shown in FIG.3C is utilized. This device essentially has disadvantages in that thesignal output is unstable in its amplitude and period since the twomicrowave signals overlap in their phases. Especially, with regard tothe device shown in FIG. 3C, the waveshape of the reflected microwavesignal changes as shown in FIGS. 5A to 5C according to respectivepositions d₁, d₂ and d₃ of the metal plate 23 as shown in FIG. 6, thusdisadvantageously generating non-effective signals. In brief, thereflected microwave signal is apt to change according to the particularthickness of the semiconductor material 10 to be measured, the positionof the metal plate 23, the thickness of the measurement table 21, thepositional relationship between the measurement table 21 and thewaveguide 8, and the like, thus generating data of little reliability.

It is noted that the amount of the reflected microwave signal is verysmall when a short lifetime of the semiconductor chip is measured, sothat the reliability of data deteriorates because that the microwavesignal is amplified through an amplifier and the lifetime data iselectrically delayed.

From still another standpoint the prior art is problematic, in thataccording to the conventional inspection method a contact breakageinspection must be carried out for the semiconductor chips or material.While the conventional process for measuring the lifetime of thesemiconductor material has considerable positive achievements concerningthe measurements of defective crystal lattices, metal taint O₂ swirlsand the like, it has problems in the inspection of very small metaltaints and surface and bulk lifetimes. Accordingly, the DLTS method of abreakage type has been used unavoidably to detect very small metaltaints.

It is apparent from above that it has not been possible to detect verysmall metal pollutants in semiconductor chips using a non-contactmethod.

SUMMARY OF THE INVENTION

The present invention is provided considering the above situation. It isaccordingly a primary object of the present invention to provide amethod for measuring the lifetime of a semiconductor material which candetect crystals existing in the material with a high reliability.

It is another object of the present invention to provide a method and anapparatus for the lifetime measurement of the semiconductor materialwhich is improved in measurement reliability through improved datareproducibility and adaptable to the semiconductor material to bemeasured.

It is still another object of the present invention to provide a methodand an apparatus for measuring the lifetime of the semiconductormaterial by detecting the very small metal taints in the semiconductormaterial or chips.

According to one aspect of the present invention, for achieving theobjects described above, there is provided a method for measuring thelifetime of a semiconductor material which includes positioning a metalface on which microwave energy passing through the semiconductormaterial reflects so as to be opposed to the semiconductor material,determining a particular distance between the semiconductor material andthe metal face, such a distance making the effect of a microwave energyportion reflected on the metal face to another portion of the microwaveenergy smallest positioning a non-metal material or member having athickness identical to the distance in a space between the semiconductormaterial and the metal face, and finely adjusting the distance betweenthe wave detector or waveguide irradiating the microwave energy and thesemiconductor material.

According to another aspect of the present invention, there is provideda method for measuring the lifetime of a semiconductor material usingequivalent distribution circuit characteristics of a waveguide which isused for irradiating microwave onto the semiconductor material variable,shifting regions where the measurement is impossible due tocharacteristics outside of a measurable area on the semiconductormaterial, and conducting the measurement within the region of theequivalent distribution circuit characteristics which are linearized.There is provided an apparatus for measuring the lifetime of thesemiconductor material including an adjusting means on the waveguide forirradiating the microwave onto the semiconductor materials for makingthe equivalent distribution circuit characteristics thereof variable.

According to still another aspect of the present invention, there isprovided a method for measuring the lifetime of a semiconductor materialwhich includes changing the temperature of the semiconductor materialwhen a lifetime of the semiconductor material is measured.

The nature, principle and utility of the present invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing the structure of the prior artapparatus for measuring the lifetime of a semiconductor material;

FIG. 2 is an explanatory view for describing the principle of the priorart lifetime measurement;

FIGS. 3A to 3C and FIG. 6 are respectively views depicting aconventional process for measuring the lifetime of the semiconductormaterial;

FIGS. 4A and 4B are views for describing characteristics of thewaveguide;

FIGS. 5A to 5C are graphs showing signal waveforms of the reflectedmicrowave energy, respectively;

FIG. 7A is a view depicting a feature of the lifetime measuring of theapparatus for carrying out the measurement method according to thepresent invention;

FIG. 7B is a view depicting an enlarged view of the encircled portionshown in FIG. 7A;

FIG. 8 is a graph showing the change of the intensity of reflectivemicrowave energy;

FIG. 9 is a structural view showing an apparatus according to thepresent invention;

FIG. 10 is an enlarged sectional view of a waveguide used in the presentinvention;

FIG. 11 is a graph showing characteristics of reflected microwavesignals of the present invention;

FIG. 12 is a view depicting a preferred embodiment of the measurementapparatus according to the present invention; and

FIGS. 13A to 13D are respective graphs showing the measurement dataobtained by the measurement apparatus according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the method for measuring the lifetime of asemiconductor material and the apparatus therefor according to thepresent invention will be explained with reference to the drawings.

According to the apparatus of the present invention, as shown in FIG.7A, a quartz glass plate 24 is placed on a metal table 25, and asemiconductor material 10 to be measured is placed on the quartz glassplate 24. A more detailed view is shown in FIG. 7B.

In the operation of the measurement apparatus shown in FIGS. 7A and 7B,a portion of the microwave energy irradiated from a waveguide 8 (foroutputting and receiving microwave energy) reflects on the semiconductormaterial 10 and another portion of the microwave energy passes throughthe semiconductor material 10. A portion of the microwave energy whichhas passed through the semiconductor material 10 reflects on the quartzglass plate 24 and another portion of this microwave energy passesthrough the quartz glass plate 24 and reaches the surface of the metaltable 25. It is understood that the reflected microwave energy receivedby the waveguide 8 is mainly that which has reflected from thesemiconductor material 10 and the metal table 25. The phasic relationbetween these two reflected microwave energy is determined by thedistance between the semiconductor material 10 and the metal table 25.Accordingly, it is possible to determine this distance using the effectof the portion of the microwave energy reflected from the metal table 25to the portion of the microwave energy reflected from the semiconductormaterial 10 by continuously checking the phasic relation. When thethickness of the semiconductor material 10 to be measured is previouslydetermined, the distance between the semiconductor material 10 and themetal table 25 is controlled by adjusting the thickness (for example, 2to 3 mm) of the quartz glass plate 24. Accordingly, it is possible tomaximize the intensity of the effective reflective microwave energy.Further, when the thickness of the semiconductor material 10 varies alittle (for example, it is shown by P or Q), the position of thewaveguide 8 for outputting and receiving the microwave is finelyadjusted to instantly obtain the maximum reflective microwave energy asshown in FIG. 8.

According to the preferred embodiment of the measurement method of thepresent invention, the metal plate 23 shown in FIG. 3C is used and aquartz glass plate having a fixed-thickness is introduced. The quartzglass plate is employed in the preferred embodiment as a non-metalmaterial of the measurement table 21 in FIG. 3C. However, it is apparentthat any non-metal material other than quartz glass may be used in themethod of the present invention.

It is noted that the method for measuring the lifetime of thesemiconductor material according to the present invention enables anincreased output precision by more than several fold in comparison tothe conventional measurement method and a constant generation of highlyreliable lifetime signals having no-strain through the reflectivemicrowave energy. When the semiconductor material to be measured by thepresent invention is the widely employed CZ-silicon, the signalscontaining the lifetime information can be treated without applying anyamplifying steps, can enjoy a wide range of measurable proportionalresistivities and can result in a significantly improved S/N ratio. Inaddition, notwithstanding the relatively easy and simple treatment ofthe output signals, it is possible to obtain an improved reliability,economy and maintenance.

In FIG. 9, the microwave energy oscillated by a microwave oscillator 1is directed to a waveguide 8 via a magic tee 4 and irradiated onto asemiconductor material (not shown) which is an object of themeasurement. The microwave energy is reflected by the semiconductormaterial to return to the waveguide 8, passed through the magic tee 4and detected by a detector 7. The waveguide 8 is provided with a stubtuner 12.

The stub tuner 12 has a structure which is shown in the enlarged view ofFIG. 10 wherein the distance D between three screws 13₁, 13₂ and 13hd 3is determined by the frequency of the microwave energy to be used. Thedistribution circuit of the waveguide 8 may be made variable byproviding the stub tuner 12 on the waveguide 8 and by adjusting thelengths l₁, l₂ and l₃ inserted within the waveguide 8. The abovearrangement can also transform the characteristic curve of the reflectedmicrowave signals from the curve a denoting the arrangement without thestub tuner (to position where measurement is impossible being at Z₀₁) tothe curve b as shown in FIG. 11. The point where a measurement isimpossible may be avoided for almost all materials by setting theresistivity, for example, at 100 Ωm in the case of a Si-wafer.Accordingly, the reflected microwave signals are outputted as an idealwaveform as shown in FIG. 5A. The curve b in FIG. 11 is improved toassume a relatively linear form relative to the non-linearcharacteristics in the region extending toward a point Z₀₂, and theamplitude variation or distortion of the reflected microwave can also berestricted.

Although a stub tuner is used as the means to make the equivalentdistribution circuit of the waveguide variable in the above embodiment,such means is in no way limited to the above and various modificationsare possible, without departing from the scope of the appended claims.

As described in the foregoing, the method and apparatus for measuringthe lifetime of the semiconductor material according to the presentinvention is highly effective since it can measure all the semiconductormaterials to obtain accurate reflected microwave signals, it cansignificantly enhance the overall measurement reliability as well asdata reproducibility and it can realize a flexible measurementarrangement.

As shown in FIG. 12, a heat-resisting or refractory member 35 is placedon an X-Y stage 36 and a heater 37 is buried in the upper portion of therefractory member 34. On the refractory member 34, a non-metalrefractory plate 33 is placed. The semiconductor material 10 to bemeasured is placed on the system consisting of the non-metal refractoryplate 33 and the refractory member 34 and the X-Y stage 36. Inoperation, microwave energy irradiates through a waveguide 8 (foroutputting and receiving the microwave energy) which is placed above thesemiconductor material 10 and excitation rays of wavelengths λ₁ and λ₂are outputted from the laser diodes 9₁ and 9₂. The semiconductormaterial 10 which is polluted by metal taints is gradually heated by theheater 37 embedded in the refractory member 34 then a lifetime of thesemiconductor 10 is measured using the reflective microwave energypassing therethrough. The measurement results as shown in FIG. 13Adepict large changes in the lifetime of the semiconductor material at acertain temperature. This phenomenon is generated because the energylevels of very small metal taints contained in the silicon materialapproach those of electrical conductors when the silicon is heated andthe excited electrons are apt to disappear. FIG. 13B shows an example ofthe lifetime changes of measurement data in which graphed on theabscissas is the inverse temperature 1/temperature (1/T) and on theordinates is lifetime τ. Measurement data was obtained on a sample a ofa semiconductor material having metal diffused and a second sample bhaving no metal. The lifetime of the second sample b is lengthened whenthe temperature of the semiconductor material 10 exceeds a certainlevel. However, the lifetime of the first sample a doesn't extend asmuch, generating a large difference between the lifetimes of the twosamples. The above result has a correlation with the peak in themeasurement data obtained the DLTS method as shown in FIG. 13C.

By previously determining on an experimental basis a relation betweenthe temperature 1/Ta shown in FIG. 13B and the temperature Tb shown inFIG. 13C, according to the non-contact and non-destructive method formeasuring the lifetime of the semiconductor material 10 of the presentinvention, it is possible to judge an existence of very small metaltaints and to determine the type of such a metal. It is possible topresume that almost all of the pollutants in the semiconductor materialconcentrates in the surface of the semiconductor chip, so that heatingthe semiconductor chip and measuring its lifetime results in a lifetimeof the much polluted surface of the semiconductor material and anotherlifetime of the little polluted interior of the semiconductor chip, andthus a separative analysis of the surface and bulk lifetimes is possibleat a high S/N ratio.

While the preferred embodiments employs a heater as the heating means asherein disclosed, it is to be understood that other forms of heatingmight be adopted.

By measuring the lifetime of the semiconductor material after it iswarmed according to the present invention, it is possible to determinethe existence of fine heavy metal taints which are identifiedconventionally only by destructive type methods, and to separatelyevaluate the surface recombination velocity (surface lifetime), thusobtaining an advantageous lifetime measurement system.

It should be understood that many modifications and adaptations of theinvention will become apparent to those skilled in the art and that theinvention is intended to encompass such obvious modifications andchanges in the scope of the claims appended hereto.

What is claimed is:
 1. A lifetime measuring method for semiconductormaterials having resistivity values within a predetermined measurementrange, said method comprising:supplying energy to a semiconductormaterial to produce carriers in said semiconductor material; irradiatingmicrowave energy from a waveguide onto said semiconductor material toobtain reflected microwave energy; varying equivalent distributioncircuit characteristics of said waveguide such that a variation of amagnitude of said reflected microwave energy relative to a resistivityof said semiconductor material is substantially linear within saidpredetermined measurement range; and, detecting said reflected microwaveenergy and measuring an attenuation of said produced carriers to obtaina lifetime measurement of said semiconductor material.
 2. A method asrecited in claim 1, wherein said varying step includes positionallyadjusting plural stub tuners located within said waveguide.
 3. A methodas recited in claim 2, further comprising providing a distance betweensaid plural stub tuners within said waveguide which is in accordancewith a frequency of said microwave energy.
 4. A lifetime measuringapparatus for semiconductor materials having resistivity values within apredetermined measurement range, said apparatus comprising:supply meansfor supplying energy to a semiconductor material to produce carriers insaid semiconductor material; a waveguide for irradiating microwaveenergy onto said semiconductor material to obtain reflected microwaveenergy; waveguide tuning means for varying equivalent distributioncircuit characteristics of said waveguide such that a variation of amagnitude of said reflected microwave energy relative to a resistivityof said semiconductor material is substantially linear within saidpredetermined measurement range; and, means for measuring an attenuationof said produced carriers in accordance with characteristics of saidreflected microwave energy to obtain a lifetime measurement of saidsemiconductor material.
 5. A method as recited in claim 4, wherein saidwaveguide tuning means includes plural stub tuners located within saidwaveguide.
 6. A method as recited in claim 5, wherein a distance betweensaid plural stub tuners within said waveguide is in accordance with afrequency of said microwave energy.